The delta-alpha high temperature bonding process (also sometimes referred to as the “delta-alpha diffusion bonding process”) can be used to bond metallic component layers of hollow axisymmetric structures for use in modern aircraft. Such structures can include an open cell or “honeycomb” metallic core and a metallic face sheet covering each opposed face of the core, for example. The high temperature bonding process generally includes compressing two or more layered metal components together at an elevated temperature with a bonding alloy disposed between the layered components. The process results in fused joints between the components that secure the layered components together.
The delta-alpha high temperature bonding process has been developed to provide a method of compressing layered shell components together as the layers are heated to a bonding temperature. The term “delta-alpha” refers to a difference (commonly designated by the Greek character delta “Δ”) in coefficients of thermal expansion (or alpha “a”). Such a process is known for bonding axially-symmetric metallic structures, such as multi-layered cylindrical or conical shells. One embodiment of the delta-alpha high temperature bonding process is generally described in U.S. Pat. No. 4,429,824, assigned to Rohr, Inc., for example.
To bond a composite shell having a simple cylindrical or simple conical shape using the delta-alpha high temperature bonding process, layered metallic components to be bonded (such as sheets of titanium honeycomb material, titanium face sheets, for example) can be positioned around a one-piece inner mandrel constructed of a material having a relatively high coefficient of thermal expansion. The outer surface of the inner mandrel is configured to substantially conform to the final shape of the inside of the cylindrical or conical shape of the composite shell. A layer of a suitable bonding alloy is disposed between the layered metallic components at locations where the components are to be bonded together. As the layered components and mandrel are heated to an elevated temperature, the difference between coefficients of thermal expansion of the layered components and the mandrel causes the inner mandrel to thermally expand more than the layered components. As a result, the expanded mandrel circumferentially stretches and radially presses on the layered shell components, thus compressing the layered components together. When the metal components are compressed together and heated to a sufficiently high temperature for a sufficient time, the bonding alloy fuses the compressed layers together. As the layered components and mandrel cool, the mandrel contracts in size, and the one-piece inner mandrel can be extracted from one end of the bonded shell, thereby releasing the bonded shell from the mandrel. The inside of the resulting bonded multi-layer shell corresponds in shape to the cylindrical or conical shape of the outer surface of the one-piece mandrel.
Though the delta-alpha bonding process described above can be used to bond multi-layer shells having relatively simple cylindrical or conical shapes, forming axisymmetric composite shells having complex curvatures by the delta-alpha bonding process presents special challenges. As used herein, the term “complex curvature” means having one or more concave and/or convex curvatures, wherein at least one intermediate diameter of the shell is either larger or smaller than both diameters at the ends of the shell. As described above, after bonding a cylindrical or conical shell, the cylindrical or conical shape of the one-piece inner mandrel and the bonded shell permit the inner mandrel to be extracted from at least one open end of the shell. For shells having non-cylindrical and non-conical complex shapes and substantial convex and or concave curvatures, however, the inner mandrel would become entrapped within the bonded shell structure such that a one-piece mandrel could not be extracted from an open end of the shell. One solution to this problem is to produce the shell in two or more generally conical or generally cylindrical sections using the bonding process described above, and then joining the sections together end-to-end to form a complete shell. Such a multi-section composite shell typically includes at least one intermediate girth seam around the shell's circumference.
Composite shells like those described above can be used as components of modern aircraft engines. For example, such a composite shell can form at least a portion of an engine's exhaust center plug. FIG. 1 shows one example of a typical engine exhaust center plug 10. The center plug 10 includes a center body shell 12 and a tail cone 14. The center body shell 12 is joined to the tail cone 14 along a circumferential girth seam 16. The center body shell 12 has a complex curvature with a maximum diameter at highlight 18 that is larger than the diameter at either end of the shell 12. As discussed above, existing methods of bonding a composite shell having complex curvatures using the delta-alpha bonding process dictate that the center body shell 12 must be constructed in at least two longitudinal sections, including a forward shell portion 12A, and an aft shell portion 12B. The substantially conical geometry of the forward shell portion 12A permits an inner mandrel portion of corresponding shape used during the delta-alpha bonding process to be extracted from an aft end of the shell portion 12A. Similarly, the substantially conical geometry of the aft shell portion 12B permits an inner mandrel portion of corresponding shape used during the delta-alpha bonding process to be extracted from a forward end of the aft shell portion 12B. The forward and aft shell portions 12A, 12B can then be joined along a girth seam 16 as indicated in FIG. 1 by a dashed line. The shell portions 12A, 12B then can be joined together along the girth seam 16 using connecting hardware and/or welds in a manner known in the art.
Unfortunately, a multi-piece shell 12 like that described above has several disadvantages. First, when connecting hardware is used to join the shell segments 12A, 12B, the number of parts required to construct the shell is not minimized. In addition, the connecting hardware adds weight to the shell assembly 12, and extends the time and cost required to produce the shell 12. Furthermore, welding the segments 12A, 12B together along the girth seam 16 adds both cost and time to the production of the shell 12. Thus, producing two segments 12A and 12B and then joining the segments 12A, 12B together adds to the overall time and cost required to produce the shell 12. Accordingly, there is a need for a method of producing a composite shell having complex curvatures in a single piece, thereby eliminating the need for connecting hardware or girth seam welds, minimizing the overall weight of the resulting axisymmetric structure, reducing the number of parts, and reducing the production time required to produce such axisymmetric composite structures having complex curvatures.